Ring laser gyroscopes utilizing counterrotating (i.e. counterpropagating) light beams are well-known. An example is shown in U.S. Pat. No. 4,006,989 which issued Feb. 8, 1977 to Keimpe Andringa for a "Laser Gyroscope". These devices are used for measuring rotation rates about an axis perpendicular to the plane of the ring laser resonant cavity by detecting the beat frequency which occurs due to a frequency difference between the counterrotating beams resulting from the rotation. However, for the ring laser gyroscopes to function at low rates of rotation, frequency locking or lock-in must be overcome. This phenomenon occurs when two traveling waves propagating in opposite directions in a resonant cavity at slightly different frequencies are pulled toward each other to combine in a single frequency standing wave. To avoid lock-in, the frequencies of the counterrotating waves must be sufficiently separated such that pulling together does not occur. The effects of lock-in are described in detail in Laser Applications, edited by Monte Ross, Academic Press, Inc., New York, N.Y. 1971, pp. 141 to 143.
The terms "light waves" or "light beams", as used herein, are not limited to only radiant energy waves with wavelengths in the visible range.
One proposal for eliminating lock-in in the ring laser resonant cavity is to use two pairs of counterrotating (counterpropagating) oppositely circularly polarized beams propagating in the cavity simultaneously. One pair consists of right circularly polarized light waves propagating in the clockwise and counterclockwise directions. The other pair consists of left circularly polarized waves which are also propagating in the clockwise and counterclockwise directions within the same resonant cavity. Such a four mode ring laser gyroscope configuration is described in detail in U.S. patent application, Ser. No. 959,237, filed Nov. 9, 1978 entitled "Four Mode Ring Laser Gyroscope With Minimum Hole Burning Competition" by Virgil E. Sanders which is assigned to the assignee of the present invention, now U.S. Pat. No. 4,213,705, issued July 22, 1980.
Disposed in the laser path of the propagating waves within the cavity are reciprocal anisotropic and nonreciprocal anisotropic dispersion elements. A reciprocal anisotropic dispersion element, such as an optical rotator made of crystal quartz, provides different delays or different optical indices to right and left circularly polarized waves. This difference in optical index due to sense of polarization is known as natural optical activity and results in an optical path length difference between oppositely polarized waves resonating within the same cavity. A nonreciprocal anisotropic dispersion element, such as a Faraday cell, presents different optical indices for light waves traveling in opposite directions such that waves traveling in the counterclockwise and clockwise directions have different delays. This delay difference produces different path lengths for light waves traveling in opposite directions. Therefore, the combination of the two types of anisotropy can be adjusted in frequency separation between resonant modes, such that all four modes resonate at different frequencies.
Separation between the resonant mode frequencies is accomplished so that the resonant frequencies of the two waves traveling in one direction are spaced between the resonant frequencies of the two waves traveling in the opposite direction. The two highest frequency modes have the same sense of polarization but opposite directions of propagation. Likewise, the two lowest frequency modes have the same sense of polarization, opposite from the sense of polarization of the other pair, and they are also counterrotating. Each pair of like-polarized modes operates as a separate two mode laser gyro. As the ring laser system is rotated about an axis perpendicular to the plane of the propagating waves, the frequency separation between the two higher frequency modes will either decrease or increase while the frequency separation between the two lower frequency modes will be oppositely affected; that is, either increase or decrease. The output beat signal resulting from combining the two lower frequency modes is subtracted from the output beat signal resulting from combining the two higher frequency modes. This produces a substantially linear representation or measure of the rotation and rotation rate of the laser system. Further, the direction of rotation is determined by monitoring one of the pairs of modes.
Because of the phenomenon known as "hole burning" the four frequencies of the four resonating modes in the cavity, in the prior art, must be substantially separated. The concept of hole burning involves the population depletion of available light emitting atoms in the gas laser medium which can emit radiant light waves at a given frequency. A laser beam sustained in a laser cavity through stimulated emission depletes the population of available light emitting atoms about that frequency and thereby results in a dip or "hole" in the laser gain vs. atom velocity curve. This hole has a certain width such that if two separate beams are operating on atom velocities very close to each other the holes overlap. As a result one of the resonant modes depletes the available atoms and will dominate the intensity of the mode operating at the adjacent frequency which will be substantially reduced or eliminated. The problems caused by hole burning were recognized by Frederick Aronowitz in his U.S. Pat. No. 3,411,849 which issued Nov. 19, 1968 entitled "Frequency Stabilized Laser". In FIG. 4 there is shown the dip or hole caused by the population depletion of the available light-emitting atoms. Hole burning is explained in detail in the text Gas Laser Technology by Douglas C. Sinclair and W. Earle Bell, Holt Reinhart and Winston, Inc. New York, N.Y. 1969, pp. 33-35.
In order to sustain all four resonating modes in the laser cavity, the frequencies of the four modes must be sufficiently separated to prevent the effects of hole burning competition. The frequency spacing must be such that there is no significant overlap between the hole burned or depleted by each resonating mode in the gain curves.
Reciprocal anisotropy is usually accomplished with a quartz crystal disposed in the laser beam path. To provide sufficient dispersion to avoid hole burning effects between the different propagating waves, in the prior art the crystal must be undesirably large. Its size contributes to thermal stresses which occur due to thermal gradients and temperature changes in the laser system and differences between coefficients of expansion of the crystal and the laser body. These stresses increase linear birefringence in the crystal, which increases coupling between different modes. Coupling here is an interaction between different waves traveling in the same direction which produces an error in the output of the laser gyro.
Typically, reciprocal and nonreciprocal anisotropy are achieved in the same element. A Faraday cell can be created by winding an electrical coil around the crystal and passing a DC current through the coil. The amount of nonreciprocal anisotropy occurring in the cell is determined by the length of the cell, the magnitude of the magnetic field, and the Verdet constant of the crystal material. A Verdet constant is defined as rotation per unit length per unit magnetic field strength. It is a material property such that different materials will have different Verdet constants associated with them.
For purposes of thermal stresses, the crystal is undesirably large. Its length, however, is very small for purposes of a Faraday cell. To achieve the required nonreciprocal anisotropy, the prior art magnetic field over the short length of the crystal must be relatively large, typically over 1000 gauss. Such high field intensity is difficult to control over the short length of the crystal element.
The above-mentioned copending patent application by V. E. Sanders teaches an approach in which the length of the reciprocal anisotropic dispersion element or quartz crystal may be reduced and the gain plasma sections may be used as a nonreciprocal anisotropic dispersion element or Faraday rotator with a greatly reduced field intensity. This concept was also discussed in a paper by V. Sanders and S. Madan and W. Chow and M. Scully, entitled "Properties of Zeeman Multi-Ocillator Ring Laser Gyro", published in the Proceedings of the IEEE 1979, National Aerospace and Electronics Conference, NAECON'79, published May, 1979. In both the patent application and the paper, a reciprocal anisotropic dispersion of as small as 10 MHz and nonreciprocal anistropic dispersion of 1 MHz are discussed.
The above-mentioned copending patent application by V. E. Sanders and the article by V. Sanders et al also describe the utilization of a laser medium, helium-neon, which includes the dual neon isotopes Ne.sup.20 and Ne.sup.22. The patent applications and paper describe how a Zeeman ring laser exhibits a large bias at zero detuning and an extreme sensitivity to cavity length tuning. The paper goes on to state that this bias may be reduced to zero by adjusting the Ne.sup.20 to Ne.sup.22 isotope ratio. In U.S. Pat. No. 4,110,045 which issued Aug. 29, 1978 entitled "Electromagnetic Wave Ring Resonator" by Irl W. Smith and Terry A. Dorschner, there is disclosed an ideal ratio of 52% Ne.sup.20 to 48% Ne.sup.22. One feature of the present invention springs from a review of the dual isotope ratios and an unexpected discovery that one ratio creats an improved Zeeman ring laser gyro.
Other parameters have also been reviewed with a goal of creating an improved ring laser gyro. The result of some of this effort has been recently reported in a paper by V. E. Sanders, S. Madan, W. Chow and M. O. Scully entitled "Beat-note Sensitivity In A Zeeman Laser Gyro: Theory And Experiment" which was published in Optics Letters, March 1980, Vol. 5, No. 3.